1. Field of the Invention
The present invention relates to a Mn—Zn based ferrite material to be used in cores for, for example, power transformers and suitable for use in high frequency bands of 1 MHz or more, in particular, 2 MHz or more.
2. Description of the Related Art
Recently, the downsizing of electric devices is remarkably developed. Accordingly, power sources mounted in various electric devices are also demanded to be further downsized. In general, when a transformer is driven with a sine wave, the magnetic flux density B is represented by B=(Ep/4.44NpAf)×107, where Ep represents the applied voltage [V], Np represents the number of turns of the primary coil, A represents the sectional area of the core [cm2], and f represents the driving frequency [Hz]. As can be seen from the above formula, for the purpose of downsizing transformers, the use of high driving frequencies for the driving frequency is effective; consequently, in these years, demanded are such high performance cores that can be used with high frequencies of the order of a few MHz.
Currently, the Mn—Zn based ferrite material is among the core materials used in the highest proportions for devices such as power transformers. This material is certainly high in permeability in the low frequency bands of the order of 100 kHz and low in loss (core loss) so as to satisfy the significant properties as a core material. However, this ferrite material is remarkably increased in loss for the driving frequencies as high as a few MHz, and hence is hardly used in practical applications in the recent circumstances that the driving frequencies are increasingly becoming higher. In relation to this problem, for example, Japanese Patent Laid-Open Nos. 6-310320 (Patent Document 1) and 7-130527 (Patent Document 2) disclose magnetic materials exhibiting low loss at 300 kHz to a few MHz, these magnetic materials being obtained by adding various oxides as additives to the Mn—Zn based ferrite materials. In this connection, under the claim that these materials are insufficient in the low-loss performance in high frequency bands, Japanese Patent Laid-Open No. 10-340807 (Patent Document 3) discloses a Mn—Co based ferrite material including Fe2O3: 52 to 55 mol %, CoO: 0.4 to 1 mol % and the balance substantially composed of MnO.
[Patent Document 1] Japanese Patent Laid-Open No. 6-310320
[Patent Document 2] Japanese Patent Laid-Open No. 7-130527
[Patent Document 3] Japanese Patent Laid-Open No. 10-340807